(1) Field of the Invention
The present invention relates generally to photosensitive devices and more particularly to an optical sensor having very low dark leakage current.
(2) Description of Prior Art
Optical sensors are utilized extensively in modem technology. Complete Guide to Semiconductor Devices, McGraw Hill, Inc., pp. 140-142, by Kwok K. Ng, discusses buried-channel charge-coupled and peristaltic charge-coupled optical devices. Traditionally, photosensitive devices are semiconductor diodes in which the light induced signal is related to the passage of photon-generated electrons and holes through the electric field region of the diode. To maintain low dark currents the diodes are reverse biased. The electrical field acts on thermally generated electrons and holes in the same way as on those that are photon-generated. Usually, light induced signals dominate those in the dark when the electron-hole generation rate due to photons dominates the thermal electron-hole generation rate. At low light intensities the differences could be small and errors could result.
A junction photo-diode, shown in FIG. 1, is a typical conventional optical sensor. Region 2 is a p-type semiconductor and region 4 is a thin n+ layer of the same semiconductor. As shown in FIG. 1, a reverse bias, V, is applied and an ammeter, 6, measures the current. The energy band diagram of the biased junction photo-diode is shown in FIG. 2. The electric field is essentially confined to a depletion region of width, W, in the vicinity of the p-n junction, as shown in FIG. 2 Prior Art. When the doping density on one side of the junction is much larger than on the other, the depletion region will predominately be on that side. The depletion width increases as the reverse bias increases. When light is incident on the surface of the n+ region, 4, and if the photon energy is sufficiently high, electron-hole pairs are generated at a rate proportional to the light intensity. For those pairs generated within the depletion region, the electrons are swept to the n+ neutral region and the holes to the p neutral region. Electrons generated in the neutral p region could diffuse to the depletion region and be swept to the n+ neutral region. Effectively, this adds a width Ln, the electron diffusion length in the p region, to W as the total width where electrons are swept by the field to the neutral n+ region. Similarly, W+Lp, where Lp is the hole diffusion length, is the width where holes are swept by the field to the neutral p region. The photocurrent is thus proportional to W+Ln+Lp times the photon induced pair generation rate. In the absence of light the current is essentially proportional to W+Ln+Lp times the thermal generation rate. Thus at low light intensities the photocurrent need not dominate the dark current.
It is important that light penetrate to the vicinity of the depletion region so that significant pair generation occurs where the created electrons and holes can be acted on by the field and thus contribute to the current. Therefor the n+ region, 4, must be thin enough to allow for this penetration. A junction photodiode can also have a thin p region disposed over an n region, with the light incident on the p region.
U.S. Pat. No. 4,965,212 to Aktik shows a junction photodiode comprised of a thin p+ type hydrogenated amorphous silicon layer disposed on a layer of n-type hydrogenated amorphous silicon. Also shown is a photosensitive diode where the p+ type layer is replaced by a thin metallic layer. Another photosensitive device utilizing hydrogenated amorphous silicon is described in U.S. Pat. No. 5,844,292 to Thierry. There the device is a p-i-n diode with the p-type, intrinsic and n-type layers being composed of hydrogenated amorphous silicon. A p-i-n photodiode operates similarly to a p-n junction photodiode, the intrinsic layer of the p-i-n diode acts in the same way as the depletion region of the p-n diode.
U.S. Pat. No. 5,114,866 to Ito et al. shows an avalanche photodiode in which an additional doped region is added to prevent edge breakdown. An avalanche photodiode is essentially a junction photodiode operated at high reverse bias where avalanche multiplication takes place. To obtain spatially uniform multiplication, edge breakdown must be eliminated.
U.S. Pat. No. 5,260,225 to Liu et al. shows a method for fabricating an infrared bolometer. The method utilizes oxide and silicon nitride layers to provide a location for the active layer, an appropriately doped polysilicon layer.
Accordingly, it is a primary objective of the invention to provide a photosensitive device with minimal dark current and where the ratio of current in light to dark current increases with increasing bias. Thus even at low light intensities the photon-induced current can be made to exceed the dark current.
The objectives of the invention are achieved by using a tunnel diode, with a wide band gap insulator providing the tunneling barrier, as the photosensitive device. For an appropriately high and thick potential barrier, currents in the dark at moderate bias are very small. Incident light energizes tunneling electrons, essentially reducing the barrier energy and increasing the tunneling probability. The affect of the bias is more pronounced on this essentially reduced barrier. Thus, whereas in conventional photodiodes the electric field act on optically and thermally generated current carriers in the same way, for tunneling photodiodes the affect of the field is more pronounced on the optically energized carriers.
According to a preferred embodiment of the invention a tunneling photodiode is fabricated by forming a p-well in an n-type substrate, forming a thin insulating layer over the surface of the p-type material, and then forming a thin n-type layer over the insulating layer. Preferably, the n and p type semiconductor material could be silicon and the insulating layer could be between about 30 to 40 angstroms of gate quality silicon dioxide.
In other embodiments of the invention the materials on either side of the insulator could be either n or p-type semiconductors or metals. The insulating layer should exhibit very low leakage in the dark and be reliable even for the thin layers, usually less then 100 angstroms, required in this invention. Gate quality insulators generally meet these requirements.